Photopolymerizable compositions and films or elements containing binder, monomer, initiator and chain transfer agent are available commercially. One important application of such photopolymerizable elements is in the graphic arts field. Elements containing such photopolymerizable layers are currently being used as electrostatic masters for analog color proofing and are considered as promising future materials to be developed for digital color proofing applications. For the analog color proofing application, a photopolymerizable layer is coated on an electrically conductive substrate and contact exposed with an ultraviolet (UV) source through a halftone color separation negative. The photopolymerizable composition hardens in the areas exposed with an ultraviolet source due to polymerization and remains in an unexposed liquid-like state elsewhere. The differences in viscosity between the exposed and unexposed areas are apparent in the transport properties, i.e., the unexposed photopolymerizable areas conduct electrostatic charge while the exposed areas are nonconductive. By subjecting the imagewise exposed photopolymerized element to a corona discharge, a latent electrostatic image is obtained consisting of electrostatic charge remaining only in the nonconducting or exposed areas of the element. This latent image can then be developed by application of an electrostatic toner to the surface. When the toner has the opposite charge as the corona charge, the toner selectively adheres to the exposed or polymerized areas of the photopolymerized element.
Photohardenable electrostatic masters are needed that duplicate the imaging characteristics of a printing press. Such electrostatic masters are known wherein the conductivity of both the exposed and unexposed areas can be controlled by introducing into a photopolymerizable composition an electron donor or an electron acceptor molecule that modify the electrical properties of the composition and provides a dot gain similar to that achieved by a printing press.
Although the use of photopolymerizable compositions in electrophotography has been demonstrated and many formulations can be imaged, it did not appear possible, to produce a photopolymerizable electrostatic master that was capable of producing both positive and negative images. Such results have been achieved with photohardenable elements which have a conductive support bearing a photohardenable layer comprising a polymeric binder, a compound having at least one ethylenically unsaturated group, an initiator, a photoinhibitor and at least one sensitizing compound. Positive and negative images are achieved depending on the exposure sequence and exposure wavelength. Such elements are extremely useful because a single element will satisfy the proofing needs of all printers regardless of whether they work with negative or positive color separations. A disadvantage of these elements is that they require two exposures to provide a positive-working electrostatic master.
High resolution, photosensitive electrostatic masters are known which upon a single imagewise exposure form conductive exposed image areas, the master comprising an electrically conductive substrate bearing a layer of a photosensitive composition consisting essentially of
(A) organic polymeric binder, PA1 (B) a hexaarylbiimidazole photooxidant, PA1 (C) a leuco dye that is oxidizable to an ionic species by the photooxidant, PA1 (D) a nonionic halogenated compound, and PA1 (E) a compatible plasticizer. PA1 (1) an electrically conductive substrate, and PA1 (2) a layer of photosensitive composition consisting essentially of PA1 (1) an electrically conductive substrate, and PA1 (2) a layer of photosensitive composition consisting essentially of
However, these masters have been found to have unsatisfactory environmental latitude.
The electrostatic properties of photosensitive masters change considerably with small variations in ambient temperature around room temperature (RT). Relatively small changes in humidity at these temperature conditions also affects electrostatic properties. For example, the discharge rates of the photosensitive layer increase with a rise in temperature. Changes in the discharge rate with ambient temperature result in degradation of print quality as well as unacceptable dot gain and dot range. Lower temperatures (RT-5.degree. C.) show lack of shadow dots while at higher temperatures (RT+5.degree. C.) highlight dots and dot gains diminish.
It has now been found that a photosensitive electrostatic master having improved environmental latitude can be made wherein the above disadvantages are substantially overcome by introducing into the photosensitive composition forming the photosensitive layer a blend of binders, at least one binder having a relatively higher glass transition temperature (Tg) than at least one other binder present. Environmental latitude may be likewise improved by introducing two or more compatible plasticizers into the photosensitive composition. The improved photosensitive electrostatic master exhibits good image quality, electrical properties and temperature stability.